1. Field of the Invention
The present invention relates to a semiconductor device containing a charge pump type step-up circuit.
2. Description of the Related Art
A prior art semiconductor device containing a charge pump type step-up circuit is disclosed in, for example, JP-H9-191571-A. When this prior art semiconductor device is mounted on a wiring board for a suitable electronic product, a step-up capacitor and a smoothing capacitor are provided on the wiring board as a part of the charge pump type step-up circuit so that an operation of the charge pump type step-up circuit can be ensured. Note that both the step-up capacitor and the smoothing capacitor are generally external to the semiconductor device.
The charge pump type step-up circuit includes a charge pump circuit having an input line to which a power supply voltage is applied, and an output line through which an output voltage is output. The step-up capacitor is connected to the charge pump circuit at the terminals thereof so as to be charged with the power supply voltage which is stepped up to a stepped-up voltage to be input through the output line. On the other hand, one terminal of the smoothing capacitor is grounded, and the other terminal of the smoothing capacitor is connected to the input line through a diode-connected MOS transistor.
In starting of the charge pump type step-up circuit, the smoothing capacitor is initially charged through the diode-connected MOS transistor until the charged voltage of the smoothing capacitor reaches the power supply voltage. As soon as the charged voltage of the smoothing capacitor reaches the power supply voltage, an operation of the charge pump circuit is substantially commenced.
In the operation of the charge pump circuit, a first terminal of the step-up capacitor is grounded, and a second terminal of the step-up capacitor is charged to the power supply voltage. Then, while the first terminal of the step-up capacitor is pulled up by the power supply voltage from the zero voltage to the power supply voltage, the second terminal of the step-up capacitor is pulled up by the power supply voltage to a voltage twice the power supply voltage, so that the voltage twice the power supply voltage is output as the output voltage through the output line. Thus, it is possible to immediately and stably output the stepped-up voltage through the output line, due to the initial charging of the smoothing capacitor.